Nowadays the emerging broadband wireless access technologies face long-term challenges to properly address the air link channel limitations with the growing demand on services, fast mobility and wide coverage. One of the most demanding and challenging scenarios is the high mobility scenarios, especially scenarios that matches the (high speed) railway domain.
Up to now, high speed railways, which generally refer to segments or sections of lines with the highest operation speed higher than 250 km/h, have been deployed in many countries such as Germany, Italy, Span, France, Japan, China, etc. The high speed railway domain introduces quite specific and challenging requirements.
The target customers of high-speed railways are people taking business trips and pleasure trips. Business travelers need to communicate with their business partners from time to time, while pleasure travelers show keen interest in entertainment services provided in the rail car. Meeting these requirements is of importance for the business of both the wireless network operators and the high-speed railway operators.
High-speed trains have a wholly-enclosed structure, and metal reflective glass is used for some models, with penetration loss reaching 24 dB or higher. This poses challenges on e.g. network deployment and cell/antenna configuration etc.
While high-speed users pass through multiple cells in a very short time, a mobile phone often cannot complete handover/cell change/cell reselection before the old cell is out of synch, leading to call drops. The network deployment and handover/cell change/cell selection procedure must be optimized to overcome this problem.
Currently, there exist several solutions targeting the problems mentioned above:
Firstly, a private network is deployed to address this problem, as described in Hu Hui's “WCDMA network coverage planning method for high speed railway”. Particularly, by adopting the private Radio Base Station (RBS)/cell and/or carrier dedicated for the high speed railway, the private network specific for high speed railway can be optimized and high speed railway customers can be served specially. This can also minimize the impact on the current public network serving thus simplifies the optimization procedure.
Secondly, network deployment optimization is employed which involves following several aspects: (1) Deploying RBS a proper distance to the railway (e.g. 500 m). A too short distance may increase penetration loss, while a too long distance may decrease cell coverage and increase dropping probability due to handover/cell change/cell reselection failure; (2) Inter-RAN and inter-LA (location area) handover/cell change/cell reselection needs more time and is thus more likely failed at high speed. Therefore it is preferable to configure the cells serving the high speed railway under the same RNC and LA (or as least as possible). Moreover, if there are multiple RNCs/LAs, the RNC and LA border should be configured in relatively low speed area, such as railway station; (3) Properly increasing the cell overlapping area so that handover/cell change/cell reselection is more likely successfully completed before the terminal moves deep into the new cell and the signal to the original cell becomes very bad.
Both approaches introduced as above may help to avoid, more or less, that the signal to the original cell becomes too bad before the completion of handover/cell change/cell reselection.
As there may be both high speed UEs and low speed UEs in the system, and the UEs speed may vary, there has been proposed to adapt the mobility parameters based on the UE speed, which implies the mobility parameters are actually on UE level. Thus, good speed estimation for the UE is the key for really benefiting from the adaptation. There are several speed estimation methods, for instance: by measuring the Doppler shift in the signal carrier frequency, either at UE or at RBS; by calculating the distance divided by the elapsed time when A-GPS is available; by estimating the frequency of cell-reselection/handover; and by estimating the variation of a measured signal.
However, though private network is deemed as an attractive solution for high speed railway, it is too costly especially considering that the wireless traffic load in a railway network is in general low. Moreover, inter-frequency handover has to be implemented between the private network and the public network which introduces additional complexity.
A proper network deployment is another important aspect for high speed railway. But this is insufficient for guarantee a successful mobility procedure at a high speed, such as a speed more than 250 km/h. For instance, a cell far from the high speed railway may occasionally provide good coverage in a small area over the high speed railway. A high speed railway UE would very likely be dropped if having this cell as the serving cell (this likely happens with a conventional fast and earlier mobility procedure). Moreover, cell coverage shrinks when traffic load increases (cell breath effect), which leads to reduced cell overlapped area, i.e. the benefit from increasing cell overlapping area reduces.
Speed based mobility parameters adaptation is also a usable solution for high speed scenario, but speed estimation is a tricky task, which is always not so inaccurate and consumes extra power and signaling (an issue especially for idle mode), while A-GPS is not always available.
Accordingly, the invention seeks to find an improved method or system to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.